In my copending application, Ser. No. 155,951 for "DETECTION OF NATURAL GAS WITHIN A CUSTOMER'S DOMAIN" filed Nov. 19, 1993, now U.S. PAT. NO. 5,437,180 issued Aug. 1, 1995 hereby incorporated by reference, I teach that the temperature of the gas stream entering the end user's internal piping system has been found to be fairly stable (at about 50 degree F) while the temperature of the end user's internal piping system (irrespective of total length) has been found to vary as a function of the ambient temperature exterior of the end user's internal piping system. Each of such temperatures can thus be used in association with a differential pressure sensing and conditioning circuit that uses a piezoresistive pressure sensor which measures differential pressures between the natural gas and ambient air entering the transducer to provide a pair of output signals, wherein the conditioned difference between these signals is related to the magnitude of flow of the natural gas based on sensed temperature changes at measured by first and second thermistors.
In such a sensor, there is provided four piezoresistive elements in or on a flexible diaphragm. Two ports are provided to provide ambient air and natural gas samples. When the diaphragm is flexed because of differential pressure, a stress is placed on the piezoresistive elements, causing them to change resistance. The resistance of such elements at any one time, is an accurate indication of differential pressure. But since both the resistance of the piezoresistive elements and its sensitivity to change as a function of stress, are dependent on a clean interface for natural gas passage, contamination due to impurities in the natural gas can unduly affect accuracy of results.
Consequently, there is clear necessity that the capacity of the filter assembly depicted and described in my above-identified application, not be exceeded. The prior art has not recognized the consequences of non-monitoring of filter condition during the above circumstances.